UniFirst Corporation is a uniform rental company based in Wilmington, Massachusetts, United States, that manufactures, sells, and rents uniforms and protective clothing. UniFirst employs more than 12,000 people and has over 230 facilities in the United States, Canada, and Europe, including customer service centers, nuclear decontamination facilities, cleanroom locations, distribution centers, and manufacturing plants.

UniFirst was founded in 1936 by the Croatti family, under the name of National Overall Dry Cleaning Company. The company began in a horse barn that had been converted into a makeshift laundry and its equipment consisted of a single washing machine and a delivery truck. It served Boston area factory workers and other laborers, whose heavy soiled work clothing needed to be cleaned frequently. The National Overall Dry Cleaning Company was incorporated in Massachusetts on October 6, 1950.

In the 1980s, UniFirst was sued by residents of Woburn, Massachusetts in a class-action lawsuit. The residents alleged that Unifirst, along with two other firms, had released pollution that had leaked into the water supply, and that this was a cause of increased instances of leukemia in the town. UniFirst settled with the residents without going to trial, for a sum of one million dollars. This episode was featured in the non-fiction book A Civil Action by Jonathan Harr. As of 2009, UniFirst's enivronmental record has improved; it has received awards for its water treatment processes from the Missouri Water Environment Association and the Water and Wastewater Utility Special Service Division of Austin, Texas, among others.

In 1991 Ronald Croatti became the chief executive officer of the company. He continued his rise in 1995, when he became the company president, and again in 2002, when he became the chairman of the board. In 2011, UniFirst featured in an episode of the reality television series Undercover Boss.

UniFirst supplies uniforms and protective clothing, as well as restroom and cleaning products such as floor mats, mops, air fresheners and soap. Products that it manufactures in-house include work shirts, work pants, outerwear, and flame-resistant work apparel. It also manufactures a majority of the garments it places in rental programs.

UniFirst subsidiary companies include Green Guard, UniTech Services Group, and UniClean. Green Guard is a corporate supplier of first aid equipment; UniTech provides laundering and decontamination services to the nuclear industry; and UniClean supplies clothing and services related to cleanrooms. UniFirst also has a Canadian uniform rental subsidiary called UniFirst Canada.